442 CHEMICAL ENGINEERING rants and requires their use. Chromel " A" may be used continuously up to 1,200°C. The tubes are heavy enough to withstand considerable mechanical strain. It has not been possible as yet to draw these alloys into tubes. The material cannot be readily machined, but when necessary can be finished off on an emery wheel. It is also possible to cut threads for pipe fittings. In order to economize on the use of these materials, iron tubes are frequently employed having short tubes of nichrome or chromel welded to them. Only the part of the protecting tube exposed to the furnace need be of the expensive alloy. In processes carried out at low temperatures where either iron tubes or chromel and nichrome tubes may be employed, experiments should be performed to determine the relative value of these tubes in life in hours per dollar of cost. There are practically no data available in this regard. In heat-treating furnaces and carbonizing furnaces, chromel " A" is often used. Chromel'.' C " and nichrome last many months in lead baths. Chromel and nichrome do not volatilize so readily as iron. Base-metal couples are thus better protected by these tubes than when iron or steel is employed. Chromel "A" contains practically no iron. Chromel "C" and nichrome have a rather high iron content. Graphite tubes afford an excellent protection to quartz or porcelain tubes on rare-metal couples, and are frequently used with, base-metal couples for molten metals. Porcelain encased in a sheath of graphite can be used in molten aluminum. Platinum couples must be thoroughly protected against the vapors distilled from graphite or carbon and from the reducing atmosphere present near heated graphite. Fireclay outer protecting tubes are used for protection in kilns, glass and steel furnaces, annealing ovens, etc. Usually they .are mounted vertically in the top of the furnace and may be cemented in place. Small fireclay insulating tubes are used on base-metal couples. Corundite consists of emery with a plastic clay binder. It is used in ceramic and glass industries for outer protection tubes. Natural corundum usually contains a large amount of iron. The artificial product, fused AM) 3, known under the trade name of alundum, is practically free from. iron. This material is very desirable for protecting rare-metal couples and for use also as outer protecting tubes. The tubes are made from ground A12O3 mixed with a clay binder. The more refractory the tubes, the less impervious to gases they become. The inner protecting tubes are glazed in order to reduce the porosity and the glaze is coated with an outer layer of alundum. This method of preparation permits the use of the tubes at temperatures above the softening point of the glaze. Such tubes are serviceable up to 1,400°C. Outer protecting tubes without glazing are made to withstand temperatures up to 1,550°C. and even higher. Alundum is mechanically strong and resists temperature changes much better than porcelain. Pure nickel is frequently used for cyanide "baths. In an oxidizing atmosphere a thick tough coating of the oxide forms which does not readily scale. The oxide thus affords protection against further corrosion. Seamless-steel and wrought-iron tubes are usually furnished with base-metal couples. They are satisfactory for many processes up to 800 or 900°C., for example in a muffle furnace. Calorizing is a process by which the surface of a wrought-iron tube is impregnated with metallic aluminum. Calorized tubes resist oxidation better than the pure iron or steel tubes. Duriron, a high-silicon iron alloy, is sometimes used at lower temperatures as a protection against acid fumes. When subjected to sudden temperature changes the material may fracture and some observers report it as extremely brittle.